1. Field of the Invention
This invention relates to an intake passage for an internal combustion engine the length of which is made variable according to the condition of the engine, and more particularly to an intake passage for an internal combustion engine in which the length of the intake passage is varied so as to enhance the output power of the engine by making the natural frequency of the intake system synchronized with the frequency of the pressure change caused by the reciprocal motion of the piston in the cylinder.
2. Description of the Prior Art
In the internal combustion engine, as the piston moves up and down in the cylinder, a pressure wave is created in the intake passage and transmits through the intake passage up to the surge tank, where it is reflected and then returns to the cylinder. This is repeated and accordingly results in a periodical pressure change in the intake passage at a frequency determined by the condition of the engine such as the engine speed. On the other hand, every intake system of the engine has its natural frequency determined by the volume of the intake passage and the cylinder. It is known that the output power of the engine can be enhanced by making the above mentioned both frequencies synchronized with each other.
In order to obtain a high output power of the engine over a wide range of engine speed, it is known to make the length of the intake passage variable according to the engine speed. This type of the intake passage is disclosed for instance in Japanese Unexamined Utility Model Publication No. 56(1981)-2023 and No. 57(1982)-22629.
These known intake passages, however, have drawbacks that they require a large space in the engine room because the length of these intake passages is made variable in the lengthwise direction. Further, these intake passages need comparatively complicated and large equipments for varying or changing the length of the intake passages.